In the United States alone, more than 650,000 people die each year of heart attacks related to coronary artery disease due to plaque build-up on the inside walls of the arteries. Various diagnostic methods have been developed to characterize and image coronary plaques in the vessel wall. These methods include angioscopic imaging, intracoronary coronary ultrasound (ICUS), and magnetic resonance imaging (MRI). Each of these methods exhibits some relative strengths and limitations/deficiencies. Optical imaging represents a promising new technology for imaging the vulnerable plaque with a level of resolution not previously achieved with the use of above conventional imaging modalities. Optical imaging can be performed with a catheter integrated with a relatively inexpensive optical fiber. A major challenge of optical imaging is that the scattering effect from the blood, especially from the red blood cells with an average diameter of 7.65 μm, blurs the image quality of the artery surface and subsurface. The absorption is small in the near infrared region from 0.8 μm to about 1.4 μm.
The use of laser surgery and advancements therein have rapidly increased in the recent years. However, laser surgery and laser cauterizing tissue normally is accompanied by the unwanted production of a cloud of smoke and vaporized particles which tend to obscure the target area (e.g., tissue).
There is thus a perceived need to develop a technique to overcome the resulting optical scattering effect in order to see through the surgical smoke and observe the target since an accurate target image is many times not possible to obtain using conventional imaging techniques.
In particular, there are many other situations in which the detection of an object present in a turbid, i.e., highly scattering, medium is highly desirable. For instance, the detection of a tumor embedded within tissue is one such example where detection of the tumor using optical imaging is difficult due to the surrounding environment that includes tissue and blood. Although X-ray techniques provide some measure of success in detecting objects in a turbid medium, they are typically not well-suited for detecting very small objects, e.g., tumors less than 1 mm in size embedded in tissue or for detecting objects in a thick or concentrated medium. In addition, X-ray radiation can present safety hazards to a person exposed thereto and thus, it would be desirable to find an alternative procedure. Ultrasound and magnetic resonance imaging (MRI) offer alternatives to the use of X-rays but have their own drawbacks and thus, all of the foregoing techniques have a number of associated deficiencies and all are particularly not well suited for a turbid medium environment.
Accordingly, it can be readily appreciated that there is an outstanding need for a high resolution optical imaging technique that is adapted for use in imaging an object in a turbid medium. Degradation of the scattering effect on non-invasive medical imaging, as well as signal transmission through atmospheric environments, are significant problems that limit the use of optical imaging techniques and the ability to obtain high resolution images.
Light propagating through a turbid medium undergoes multiple scattering, which randomizes the direction of propagation, phase, and polarization of the incident light. The image quality is degraded in this process and therefore, the resulting image is of poor quality and resolution. To reduce the effect of multiple scattering on obscuring the image, over the years various techniques have been introduced, such as time-resolved techniques, ballistic 2-D imaging through scattering wall using an ultrafast Kerr gate, optical coherent tomography, time-resolved coherent and incoherent components of forward light scattering in random media, frequency-domain techniques, nonlinear optical techniques, subsurface tumor progression investigated by noninvasive optical second harmonic tomography, optical low-coherence, Fourier space gate technique, and confocal fluorescence microscopy.
Details of the above types of techniques and systems can be found in following patents and articles: U.S. Pat. No. 5,371,368, issued Dec. 6, 1994 to Alfano et. al.; Wang, et al, Ballistic 2-D imaging through scattering wall using an ultrafast Kerr gate, Science 253, 769(1991); Huang, et al, Optical coherent tomography, Science 254, 1178(1991); Yoo, et al, Time-resolved coherent and incoherent components of forward light scattering in random media, Opt. Lett. 15, 320(1990); O'Leary, et al, Experimental images of heterogeneous turbid media by frequency-domain diffusing-photon tomography, Opt. Lett. 20, 426(1995); U.S. Pat. No. 6,208,886, issued Mar. 27, 2001 to Alfano et. al.; Guo, et al.; Proc. Natl. Acad. Sci. 96, 10854 (1999); Schmidt, et al, Imaging through random media by use of low-coherence optical heterodyning, Opt. Lett. 20, 404(1995); Dolne, et al, IR Fourier space gate and absorption imaging through random media, Lasers in the life Sci. 6, 131(1994); and Masters, et al, Ultraviolet confocal fluorescence microscopy of the in vitro cornea: redox metabolic imaging, Appl. Opt. 32, 592(1993), all of which are hereby incorporated by reference in their entireties.
However, the above techniques are limited by small imaging depth and the techniques are difficult to implement due to the sophisticated mathematical problems that are encountered in implementation or due to complicated experimental setups, etc. and some polarization imaging techniques have been developed and can be adapted for use in the present invention, See e.g., U.S. Pat. No. 5,847,394, issued Dec. 8, 1998 to Alfano et al; Demos, et al.; Optical polarization imaging, Appl. Opt. 36, 150 (1997); Lewis, et al, Backscattering target detection in a turbid medium by polarization discrimination, Appl. Opt. 38, 3937 (1999); and Pernicka, et al, Improvement of underwater visibility by reduction of backscatter with a circular polarization technique, Appl. Opt. 6, 741 (1967), all of which are incorporated herein by reference in their entireties.